lidavidm commented on pull request #9802:
URL: https://github.com/apache/arrow/pull/9802#issuecomment-809437568
Aha, the reason why scanning a fragment is empty is because it gets
constructed with an empty schema due to a spot of undefined behavior.
```cpp
InMemoryFragment::InMemoryFragment(RecordBatchVector record_batches,
Expression partition_expression)
: InMemoryFragment(record_batches.empty() ? schema({}) :
record_batches[0]->schema(),
std::move(record_batches),
std::move(partition_expression)) {}
```
This might move record_batches before we evaluate empty(), resulting in an
empty schema getting passed.
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